CD6 is an important cell surface protein predominantly expressed by human T cells and a subset of B cells, as well as by some B cell chronic lymphocytic leukemias and neurons [Aruffo et al., J. Exp. Med. 1991, 174:949; Kantoun et al., J. Immunol. 1981, 127:987; Mayer et al., J. Neuroimmunol. 1990. 29:193]. CD6 is a member of a large family of proteins characterized by having at least one domain homologous to the scavenger receptor cysteine-rich domain (SRCR) of type I macrophages [Matsumoto, et al., J. Exp. Med. 1991, 173:55 and Resnick et al., Trends Biochem. Sci. 1994, 19:5]. Other members of this family include CD5 [Jones et al., Nature. 1986, 323:346]; cyclophilin C [Friedman et al. 1993, PNAS 90:6815]; complement factor I, which binds activated complement proteins C3b and C4b [Goldberger, et al., J. Biol. Chem. 1987, 262:10065]; bovine WC-1 expressed by .tau./.delta. T cells [Wijingaard et al., J. Immunol. 1992, 149:3273] and M130 [Law et al., Eur J. Immunol. 1993, 23:2320], a macrophage activation marker.
Blocking studies using anti-CD6 monoclonal antibodies (mAbs) suggest that CD6 plays an important role in T cell development by regulating T cell adhesive interactions with thymic epithelial (TE) cells [Patel et al., J. Exp. Med. 1995 181:1563-1568]. Additional studies have shown that CD6 can function as an important accessory molecule in T cell activation. For example, certain anti-CD6 mAb are directly mitogenic for T cells [Gangemi et al., J. Immunol. 1989, 143:2439 and Bott et al., 1993 Int. Immunol. 7:783], whereas others are able to co-stimulate T cell proliferation in conjunction with anti-CD3, anti-CD2 or phorbol 12 myristate 13 acetate (PMA) [Gangenzi et al., J. Immunol. 1989, 143:2439; Morimoto et al., J. Immunol. 1988, 140:2165-2170; and Osorio et al., Cell. Immunol. 1994, 154:23]. Yet additional evidence of the role of CD6 in T cell activation comes from studies showing that CD6 becomes hyperphosphorylated on Ser and Thr residues [Swack et al., Mol. Immunol. 1989 26:1037-1049 and J. Biol. Chem. 1991, 266:7137; Cardenas et al., J. Immunol. 1990, 145:1450-1455] and phosphorylated on Tyr residues [Wee et al., J. Exp. Med. 1993, 177:219-223] following T cell activation. These and other studies implicate CD6 as an important modulator of both immature and mature T cell function in vivo, affecting both T cell activation and signal transduction.
The extracellular domain of the mature CD6 protein is composed of three SRCR domains (hereinafter designated D1, D2, and D3). D3 corresponding to the membrane proximal SRCR domain followed by a short 33-amino-acid stalk region. These extracellular domains are anchored to the cell membrane via a short transmembrane domain followed by a cytoplasmic domain of variable length [Aruffo et al., J. Exp. Med. 1991, 174:949].
Studies using CD6-immunoglobulin fusion proteins, containing selected extracellular domains of CD6 fused to human IgG.sub.1 constant domains (CD6-Rgs), led to the identification and cloning of a CD6 ligand, designated “activated leukocyte cell adhesion molecule” (ALCAM) [Wee, et al., Cell. Immunol. 1994, 158:353-364; Patel, et al., J. Exp. Med. 1995. 181:1563-1568; Bowen et al., J. Exp. Med 1995, 181:2213-2220]. ALCAM binds to domain 3 of CD6 corresponding to the membrane proximal SRCR domain [Whitney, et. al., J. Biol. Chem. 1995, 270: 18187-18190].
Studies of the role of CD6/ALCAM interactions in T cell regulation have shown that this receptor-ligand pair is able to mediate the adhesion of CD6 expressing cells to thymic epithelial cells [Bowen et al., J. Exp. Med. 1995, 181:2213]. This and other evidence suggests that CD6/ALCAM interactions are important for modulating T cell development and activation.
Although the functional characterization of CD6 remains incomplete, an anti-CD6 mAb have been successfully applied in a clinical setting to purge bone marrow of T cells and T cell precursors. The finding that patients receiving anti-CD6-treated allogeneic bone marrow had a low incidence of graft-vs-host disease coupled with high levels of engraftment [Soiffer R J, 1993, Bone Marrow Transplant; 12 Suppl 3:S7-10] led to the discovery of a small subset of peripheral blood T cells (5-6%) that are CD6 negative (Rasmussen. J Immunol 1994. 152: 527-536). Subpopulation of CD6 negative T cells displayed lower alloreactivity in MLRs compared with normal CD6+ T cells. (Rasmussen. J. Immunol 1994. 152: 527-536). Functional characterization of these CD6-negative T cells has also shown that they are unresponsive to allostimulation, but can proliferate when stimulated with phytohemagglutin (PHA). These findings further support the hypothesis that CD6 plays an important role in modulating T cell function in vivo. CD6 is also reported to be part of the immunologic synapse mediating early and late T cell-APC interaction. (Gimferrer I. J. Immunol 2004. 173: 2262-2270).
The CD6 molecule is N glycosylated with a protease sensitive site and possesses intrachain disulphide bonds. Previous reports indicated that CD6 exists in two molecular forms, a phosphorylated form of 105 kDa in resting T cells and a hyperphosphorylated form of 130 kDa in cells after protein kinase C activation by the tumor promoter, phorbol 12 myristate 13 acetate (PMA) (Osorio M, Cellular Immunology, 1994154:123-133).
U.S. Pat. No. 6,372,215 discloses antibodies and other binding agents that bind specifically to SRCR domains 3 (D3) of human CD6 (hCD6) or human CD6 stalk domain (CD6S) and inhibit activated leukocyte cell adhesion molecule (ALCAM) binding to CD6 the contents of which are herein incorporated by reference.
Cuban patent application CU 250/2006 dated 26 Dec. 2006 titled “Pharmaceutical composition comprising the anti CD6 monoclonal antibody useful for the diagnosis and treatment of Rheumatoid Arthritis” discloses that T1h binds to CD6 without inhibiting the binding of CD6 to the ALCAM ligand the contents of this application are herein incorporated by reference.
The CD6 antibody of the current invention prevents the activation of T cells by inhibiting T cell proliferation by binding to a domain independent to the domain interacting with the known ligand to CD6 namely ALCAM.
Earlier publications and patents disclose sequences of the murine anti-CD6 (IOR-T1) monoclonal and the amino acid modifications that were carried out to humanize IOR-T1 to T1h (humanized IOR-T1). U.S. Pat. No. 5,712,120 and its equivalent EP 0699755 disclose specific methods to humanize murine monoclonal antibodies and the sequence of IOR-T1 and T1h. U.S. Pat. No. 6,572,857 and its equivalent EP 0807125 disclose the sequence of IOR-T1 and T1h (humanized IOR-T1). The publication [Roque-Navarro, L., et. al., Hybridoma and Hybridomics 2003.22:245-257] discusses specific methods to humanize murine monoclonal antibodies and the sequence of IOR-T1 and T1h.
Aspects of the present invention relate to amino acid sequences of the variable region of heavy and light chain of T1h. This establishes the T1h nucleotide and amino acid sequence as expressed by the cell line used for manufacturing T1h. The monoclonal antibody of the present invention is capable of binding to domain 1 (D1) of CD6 and inhibits T-cell proliferation without interfering with ALCAM binding. The monoclonal antibody of the present invention does not induce complement dependent cytotoxicity (CDC), antibody dependent cytotoxicity (ADCC) and apoptosis in vitro.